Groundwater can be contaminated by a number of different organic and metallic compounds, causing potential threats to human health and the environment. For organic contaminants, one of the most effective methods to remediate contaminated groundwater sites is via in-situ biodegradation, where naturally occurring, or introduced, bacteria transform the contaminants to innocuous by-products. For dissolved heavy metal contaminants, chemicals are added to change the redox state of the groundwater (i.e., how aerobic or anaerobic the groundwater is) to render the contaminant immobile in groundwater.
Several beneficial biodegradation and geochemical reactions in the subsurface occur only in anaerobic or deeply anaerobic conditions, where little or no dissolved oxygen is present in the groundwater. For example, xe2x80x9chalorespirationxe2x80x9d is the most effective process contributing to the biodegradation of the highly chlorinated solvents such as perchloroethene, trichloroethene, trichloroethane, and carbon tetrachloride in groundwater (Wiedemeier et al., 1999). During halorespiration, the chlorinated hydrocarbon is used directly as an electron acceptor, not as a source of carbon, and a chlorine atom is removed and replaced with a hydrogen atom. The optimum range for halorespiration is under sulfate-reducing (moderately anaerobic) and methanogenic (deeply anaerobic) conditions (Wiedemeier et al., 1999).
Another biodegradation reaction of interest includes the anaerobic biodegradation of methyl-tert butyl ether (MTBE). Yeh and Novak (1994) stated that removal of MTBE was only associated with methanogenic. (deeply anaerobic) conditions. Mormile et al. (1994) found no biodegradation of MTBE under slightly or moderately anaerobic conditions (i.e., nitrate-reducing and sulfate reducing environments) but did see biodegradation under deeply anaerobic (methanogenic conditions). Wilson et al. (2000) performed column studies that demonstrated relatively high rates of MTBE biodegradation under deeply anaerobic conditions in both lab microcosm studies and at a field research site. Other beneficial anaerobic reactions involve the biodegradation of perchlorate, certain pesticides, and brominated compounds.
While metals do not biodegrade, some metals in groundwater become less mobile under anaerobic conditions. For example, under aerobic (oxidizing) conditions, hexavalent chromium Cr(VI), the most toxic form, is typically stable in groundwater. Under reducing (anaerobic) conditions, however, Cr(VI) is transformed to Cr(III), immobilizing it as the solid Cr(OH)3 (Deutsch, 1997). Therefore, anaerobic reactions are beneficial as they can result in a change in geochemical conditions that can reduce the toxicity and mobility of the chromium.
Most groundwater plumes originate from long-lived stationary source zones comprised of contaminated surface soils or more commonly, subsurface accumulations of non-dissolved organics (also called non-aqueous phase liquids or NAPLs). Groundwater flows through the stationary contaminated source zones carrying unwanted dissolved oxygen, nitrate, and sulfate. If dissolved oxygen is delivered to a contaminated zone, then the efficiency of anaerobic reactions will be reduced or the anaerobic reactions will stop altogether. Similar constraints on deeply anaerobic reactions occur when nitrate and sulfate are transported to the contaminated zone as clean, upgradient groundwater flows through the contaminated zone. If a reaction only occurs under deeply anaerobic (i.e., methanogenic conditions), then the transport of nitrate and sulfate to the contaminated zone will either reduce the efficiency of the reaction, or stop the reaction altogether.
Many sites have conditions that will remove all or some of the dissolved oxygen, nitrate, and sulfate. For example, a method developed by the U.S. Air Force (Wiedemeier et al., 1996) for chlorinated solvent sites classifies a contaminated groundwater zone as a Type I, II, or III site. At Type I sites anthropogenic hydrocarbons (such as fuel contaminants) are present in source zones containing NAPL in such concentrations that some or most of the interfering electron acceptors have been removed and beneficial anaerobic biodegradation reactions are on-going. A Type II site has similar conditions, except that carbon sources are not man-made but are naturally present in the flowing groundwater. A Type III site has no hydrocarbons that can turn a site anaerobic, and therefore no beneficial anaerobic reactions can occur.
To enhance beneficial anaerobic processes for the purpose of bioremediation, numerous research groups have focused on methods to increase the supply of electron donor to the dechlorinating bacteria. Bacteria use this increased electron donor supply in two ways: 1) to react with any dissolved oxygen, nitrate, or sulfate present in the contaminated groundwater zone, making the zone more anaerobic; and 2) to react with contaminants, such as chlorinated solvents. The more dissolved oxygen, nitrate, manganese, or sulfate that is present, the more competition there is for the added electron donor.
Most researchers and technology developers have focused on adding an indirect electron donor (such as lactate, molasses, mulch, edible oil, or other carbon source) that is fermented by one type of in-situ bacteria to produce hydrogen. The hydrogen is then consumed by other types of bacteria to help induce or enhance anaerobic conditions, or used directly as a substrate in anaerobic biodegradation reactions. A second, more recent method involves the delivery of dissolved hydrogen directly to the subsurface to enhance anaerobic biodegradation of chlorinated solvents (Hughes, Newell, and Fisher, U.S. Pat. No. 5,602,296, Feb. 11, 1997).
The disadvantage to both of these methods is that 1) large quantities of electron donor can be consumed by reactions with the interfering electron acceptors; 2) addition of an electron donor is expensive due to the need for injection wells, pumps, process equipment on the surface, etc.; 3) operating the system can be risky due to the potential for moving contaminants outside the existing zone of contamination via the injection of the injection fluid; 4) operating the system can be difficult due to the potential for chemical or biological clogging of the injection wells; and 5) operating costs can be high due to the need for maintaining the process equipment and electron donor injection system.
An alternative, but still unsatisfactory, approach chemically removes electron acceptors, as opposed to adding electron acceptors. For example, Rice and Koch in U.S. Pat. No. 6,001,252, Dec. 14, 1999, teach xe2x80x9cinjecting in-situ into a groundwater-saturated matrix within or upgradient of a source of the organic compound a deoxygenated aqueous solution that comprises an electron donor to facilitate reductive dehalogenation of the organic compound.xe2x80x9d This approach chemically removes one unwanted electron acceptor, dissolved oxygen, by adding a deoxygenating agent.
Addition of a deoxygenating agent has many of the same disadvantages to adding electron donors directly to the contaminated groundwater zone: 1) large quantities of deoxygenating agent may be required; 2) addition of a deoxygenating agent is expensive due to the need for injection wells, pumps, process equipment on the surface, etc.; 3) operating the system can be risky due to potential for moving contaminants outside the existing zone of contamination if the injection fluid is injected into the contaminated groundwater zone; and 4) operating costs can be high due to the need for maintaining the process equipment and electron donor injection system.
In summary, there is a need for a method that can enhance beneficial anaerobic reactions without the need for the injection of an electron donor or deoxygenating agent.
The present invention is a method for enhancing in-situ, natural anaerobic biodegradation, also called intrinsic bioremediation or passive bioremediation, of groundwater contaminants in a saturated aquifer matrix. The type of contaminated groundwater site applicable to the present invention is identified as having the correct hydrogeological and chemical parameters for remediation via enhanced biodegradation. A shielding system is constructed at a contaminated groundwater site to physically interrupt electron acceptors in upgradient groundwater from flowing into the contaminated groundwater zone. The shield may comprise a barrier wall installed in the subsurface that diverts groundwater around the treatment zone or comprise a series of groundwater pumping wells installed to remove upgradient groundwater prior to the groundwater entering the treatment zone area.
An advantage to the shielding system is that a barrier is only constructed on the upgradient part of the contaminated groundwater treatment zone rather than completely surrounding the zone. The barrier forms a stagnant area in the contamination zone area, thereby preventing electron acceptors in clean groundwater from flowing into the stagnated zone. The barrier only needs approximately 90 percent efficiency to perform correctly, which reduces cost and expands options for material selection, alignment, and construction of the barrier. Via this approach, contaminated groundwater is not removed from the treatment zone, and all remediation is performed in-situ, which removes the environmental hazards of handling contaminated water at the surface. By constructing a barrier upgradient of a contaminated zone, beneficial anaerobic reaction rates within the zone are increased and the migration of contaminants from the zone is reduced. Additional mechanical equipment or source plume manipulation is not needed to meet remediation goals. The anaerobic microbiologic activities within the source zone are enhanced passively, thereby increasing the rate of removing contaminants within the zone while minimizing operation and maintenance activities at the site.
In the preferred embodiment, chlorinated solvents dissolved in groundwater are targeted for reductive dechlorination with enhanced anaerobic treatment. MTBE is another common contaminant that biodegrades primarily under deeply anaerobic conditions. This contaminant is a treatable chemical in an enhanced treatment system. The system may also be used to treat dissolved metals in groundwater, such as chromium. Chromium, as Cr (VI), will not biodegrade in the subsurface: however, it can be immobilized as a solid after exposure to enhanced anaerobic biodegradation reactions.